The present disclosure relates to generating and processing signals in a physiological monitoring system, and more particularly to techniques for scaling filter histories or filter coefficients to attenuate filter artifacts in a received signal to improve the determination of physiological parameters by the physiological monitoring system.
Photoplethysmography (PPG) is a non-invasive, optical measurement that may be used to detect changes in blood volume within tissue of an individual. PPG signals may be used by pulse oximeters, vascular diagnostics system, and digital blood pressure detection systems. Typically, a PPG system includes a light source that is used to illuminate tissue of a patient and a photodetector that is used to detect light attenuated by the tissue of a patient. A PPG system may analyze variations in the detected light intensity that may be associated with blood volume changes proximal to the illuminated tissue. The analysis may be used to determine physiological parameters of the individual.
PPG systems often utilize a variety of filters to condition or process the output of the photodetector. Some filters comprise a filter history or filter coefficients. For example, the output of digital Finite Impulse Response (FIR) filters may be determined by filter coefficients and a history of previous input samples. The output of digital Infinite Impulse Response (IIR) filters may also depend on a history of previous outputs from the filter. During normal operation, a PPG system may adjust the intensity of the light source and change settings of gain circuitry or ambient light cancellation circuitry that processes the photodetector signal in order to amplify the signal. These adjustments may be performed in order to increase the signal-to-noise ratio, to avoid saturating an ADC (analog-to-digital converter) of the PPG system, or to save power when driving light emitters or for a number of other reasons. Changes to light intensity, gain or ambient light rejection may occur very frequently when emitters are dynamically adjusted within a cardiac cycle to save power. Some exemplary techniques for changing light intensity are disclosed in U.S. Patent Application Publications Nos.: 2013/0324809, 2013/0324855, 2013/0324856, and 2015/0173687, which are hereby incorporated by reference. In some embodiments, the filters may experience an effect similar to an effect produced by a change in emitter drive current of a system that changes emitter wavelengths. Such effects are described, for example, by U.S. Pat. No. 8,649,838, which is hereby incorporated by reference. When these adjustments in light detection gain or in light intensity occur, the filter history may simultaneously include photodetector signal samples acquired at the old level of the photodetector signal and the new level of the photodetector signal. In addition, filtering coefficients may be applied simultaneously to the old level of the photodetector signal and the new level of the photodetector signal. Such simultaneous processing can introduce filter artifacts (e.g., a step response) into the photodetector signal. These filter artifacts may negatively affect determination of physiological parameters by the PPG system.
Accordingly, the present specification discloses improved systems and methods for determining physiological parameters based on a PPG signal, where the filter history and/or the filter coefficients are scaled based on adjustments to the intensity of one or more light sources and adjustments to the settings of gain controller or ambient light cancellation circuitry in a manner designed to eliminate or attenuate filter artifacts. The disclosed systems and methods improve the operation of a PPG system by eliminating or attenuating filter artifacts that may introduce errors into algorithms for determining physiological parameters (e.g., heart rate, oxygen saturation, respiration rate, and regional oxygen saturation).